The single layer of polarized intestinal epithelial cells (IECs) which line the entire gastrointestinal tract express low levels of class II antigens constitutively, with elevated levels seen in a variety of autoimmune and inflammatory conditions including Inflammatory Bowel Disease (IBD). The long term goal of this grant proposal is to understand the mechanisms and significance of class II antigen processing by intestinal epithelial cells (IECs). We hypothesize that the class II mediated processing and presentation of luminal antigens by IECs contributes to the maintenance of CD4+ T cell compartments in the intestinal mucosa, and may contribute to the exaggerated mucosal CD4+ T responses that result in intestinal inflammation, including IBD. We have demonstrated that class II expressing IECs display unique features of class II processing and presentation related to their polarized phenotype and can function as potent antigen presenting cells to CD4+ T cells. This proposal outlines strategies to both define the molecular determinants of the polarized class II pathways operational in IEC, and to determine the biological relevance of IEC class II expression with regards to mucosal CD4+ T cell development and intestinal inflammation. We will examine the role of antigen denaturation, receptor mediated endocytosis, and the recycling of class II from the plasma membrane, respectively, on the class II antigen processing initiated from the apical or basolateral surface of polarized IEC. Experiments are proposed to define the molecular targeting signals that both direct the highly polarized expression of class II molecules at the basolateral surface and direct the localization of class II molecules in intracellular compartments in polarized IEC. Moreover, we describe the development of a novel model system in which the expression of class II molecules is targeted to IEC in either the small intestine or colon of transgenic mice. We plan to use these mice to determine the biological relevance of IEC class II expression to the development and function of normal and pathological CD4+ T cell populations in the intestinal mucosa. We anticipate that the experiments outlined in this grant proposal will highlight the IEC as an important therapeutic target in the modulation of mucosal immune and inflammatory responses.